Inorganic-organic interfaces, owing to their unique chemical and electronic properties, are playing an increasingly important role in several technologies including organic light emitting diodes (OLEDs) molecular electronics and microelectronic interconnect technology: e.g. interfaces between carbon-based low-κ dielectrics and metallic/inorganic diffusion barriers. Despite their importance, many aspects of the formation of these interfaces are not fully understood.
Self-assembly is a popular method for making highly ordered (over nm length scales), organic monolayer films on metallic and semiconductor substrates. These self-assembled organic-on-inorganic monolayers (SAMs) have been widely studied as model surfaces owing to their ease of formation and self-limiting growth characteristics. For example, alkyltrichlorosilane SAMs on silicon dioxide are formed by spontaneous reaction, adsorption and organization of a long chain molecule on the SiO2 surface, e.g. (—O-)3Si—(CH2)nX, where typically n≧8. The specificity of the reaction chemistry leaves the functional group, X, at the surface, enabling the tailoring of surface properties. These features of SAMs have made them the preferred method for tailoring the surface chemistry of inorganic surfaces.
“Inorganic-on-organic” interfaces are also important, in particular, in applications such as barrier layers (e.g. encapsulation of the aforementioned metallic interconnects), reflective coatings, and electrical contacts for both OLEDs and molecular electronics. Formation of these interfaces, however, is much less mature in comparison to “organic-on-inorganic” interfaces constructed using SAMs. To date, the inorganic component of the interface has been a metal or an oxide formed by (elemental) evaporation in vacuum, or by deposition in the liquid phase using a metal complex.
Formation of TiO2 thin films on SAMs by deposition through the liquid phase has attracted recent interest. Sukenik and coworkers established a route to the synthesis of polycrystalline TiO2 thin films by reacting TiCl4 and Ti(OCH(CH3)2)4 with alkyltrichlorosilane self-assembled monolayers bearing sulfonate and —OH functional groups respectively. Zhongdang et al. deposited TiO2 thin films from the reaction of TiCl4 with sulfonate terminated trimethoxysilane SAMs on soda glass substrates, and found Ti2+, Ti3+ and Ti4+ oxidation states in the deposited film. More recently, Niesen et al. formed TiO2 thin films from the reaction of aqueous titanium peroxide solutions with trichlorosilane SAMs with different terminal groups. They found that sulfonate terminal groups assisted in the formation of densely packed films while hydroxyl and amine terminal groups led to the formation of large islands (70-200 nm in size), which eventually coalesced into a thin film possessing distinct domains. Masuda et al. obtained site-selective deposition of TiO2 from TiCl4 and Ti(OC2H5)2Cl2 onto silanol regions created in octadecyltrichlorosilane (OTS) SAMs by UV exposure. However, deposition was not restricted to the silanol regions for 3-aminopropyltriethoxysilane (APTES) and phenyltrichlorosilane (PTCS) SAMs, which was attributed to disorder introduced by the bulky phenyl group for the PTCS SAM, and the adsorption of water on the APTES SAM. XPS revealed that the TiO2 films that were formed had significant carbon and chlorine contamination.
Vapor phase evaporative deposition of elemental metals on functionalized SAMs has also been studied. Jung and Czanderna have examined the evaporation of elemental metals onto SAMs with different organic functional end groups (OFGs). They broadly categorized the metal/OFG interactions to be .strong. (e.g. Cr/COOH or Cu/COOH) where the deposit was found to reside primarily on top of the SAM (linked to the OFGs) or .weak. (e.g. Cu/OH, Cu/CN, Ag/CH3, Ag/COOH) where the metal was found to penetrate the SAM and was bound at the SAM/substrate interface. Allara and co-workers used XPS to study interfacial chemistry and film morphology in situ during elemental evaporation of Ti on alkanethiol SAMs with different terminal groups. Elemental Ti was found to be highly reactive with the —OH, —CN, and —COOCH3 terminal groups, first forming TiOx and TiNx species at low coverages, while formation of TiCx species, possibly due to reaction with the SAM backbone, was apparent at higher coverages. These reactive end groups on the SAM yielded smaller islands and thin films with smaller roughness when compared to SAMs with less reactive end groups (i.e. —CH3), where significant 3-D growth was observed. Allara and co-workers also studied the reaction of vapor deposited aluminum with —CH3, —COOCH3 and —COOH terminated alkanethiol self-assembled monolayers on polycrystalline gold. While significant penetration of Al to the SAM/Au interface was observed for the —CH3 terminated SAM, reaction of Al with the —COOCH3 and —COOH terminated SAMs was confined to the SAM/vacuum interface.
The deposition of thin inorganic films on SAMs using organometallic precursors has received relatively less attention. The formation of Au, Pd and Al thin films by the reaction of organometallic precursors on SAMs has been examined. In the case of Au and Pd deposition on thiol-based SAMs, only spatial selectivity and thin film morphology were examined. For Al deposition from trimethylaminealane on —OH, —COOH and —CH3 terminated thiol SAMs, interfacial chemistry was examined using XPS, but an explicit examination of the kinetics of adsorption was not attempted.
There is a need for systems and methods that provide better control of the preparation and composition of thin-film inorganic materials.